With the progress of small-size, high-speed and lightweight electronic devices such as cellular phones and personal computers, the capacitor used for these electronic devices is demanded to have a smaller size, a larger capacitance and a lower equivalent series resistance (ESR).
As an example of such a capacitor, an electrolytic capacitor has been proposed, which capacitor is produced by anodically oxidizing an electrode comprising a sintered body made of a valve-acting metal powder which can be anodized such as tantalum to form a dielectric layer made of the oxide of the metal on the surface of the electrode.
The electrolytic capacitor using tungsten as a valve-acting metal and employing a sintered body of the tungsten powder as an anode body can attain a larger capacitance compared to the electrolytic capacitor obtained with the same formation voltage by employing an anode body of the same volume using the tantalum powder having the same particle diameter. However, the electrolytic capacitor having the sintered body of the tungsten powder has been unpracticed as an electrolytic capacitor due to the large leakage current (LC). In order to solve this issue, a capacitor using the alloy of tungsten and other metals has been studied and has achieved some improvement in the leakage current, but it was not enough (JP-A-2004-349658 (U.S. Pat. No. 6,876,083 B2); Patent Document 1).
Patent Document 2 (JP-A-2003-272959) discloses a capacitor using an electrode of a tungsten foil having formed thereon a dielectric layer selected from WO3, W2N and WN2, but the capacitor is not to solve the above-mentioned leakage current problem.
Also, Patent Document 3 (WO 2004/055843 publication (U.S. Pat. No. 7,154,743 B2)) discloses an electrolytic capacitor using an anode body selected from tantalum, niobium, titanium and tungsten, but it does not describe a specific example using tungsten in the specification.